1. Field of the Invention
The present invention relates to the field of rotary-wing aircraft having gimbaled rotor hubs.
2. Description of Related Art
Consumer demand is increasing for rotary-wing aircraft to provide more thrust, higher speeds, and carry heavier loads and/or heavier fuselages. For example, there is a demand for more powerful tilt-rotor aircraft. Of course, where performance criteria such as those listed above are to be increased, the functional systems of the rotary-wing aircraft must be improved to provide the desired resultant performance enhancements. The rotor hub drive system is one of the many functional systems which requires improvement in order to meet the demand for improved rotary-wing aircraft performance.
Rotor hub drive systems often are/include constant-velocity drive systems, or homokinetic drive systems, which have been in use for a very long time. There are numerous successful designs of constant-velocity drive systems for various types of rotary-wing aircraft. Constant-velocity drive systems are typically designed for transferring torque, or rotational force, from a first rotating member to a second rotating member, where the first rotating member may not be coaxial with the second rotating member. Constant-velocity drive systems are particularly well suited for use in rotary-wing aircraft as a means of transferring torque from a rotating mast to a rotor hub, especially where the rotor hub is gimbaled to the rotating mast. Two such constant-velocity drive systems are taught by Zoppitelli et al. in U.S. Pat. No. 6,712,313.
Zoppitelli et al. teach a first constant-velocity drive system where a torque-splitting mechanism (see Zoppitelli et al. FIGS. 2-6) is associated with a two-gimbal device (see Zoppitelli et al. FIGS. 7 and 8) for driving in rotation and tilting (with respect to a mast) a rotor hub. Zoppitelli et al. also teach a second constant-velocity drive system where the same torque-splitting mechanism drives a rotor hub in rotation via drive links and where the rotor hub is gimbaled to the mast by a gimbal means comprising half of a flapping thrust bearing (see Zoppitelli et al. FIGS. 9 and 10). In the second constant-velocity drive system, the differential mechanism drives the hub in rotation via drive links while the hub is connected to the mast with a tilting means comprising a flapping thrust bearing.
Referring now to FIG. 1, a tilt-rotor, rotary-wing aircraft incorporating a constant-velocity drive system as taught by Zoppitelli et al. is illustrated. Tilt-rotor aircraft 17 is shown in an airplane mode of flight operation. When aircraft 17 is in an airplane mode, wings 19 (only one shown) are utilized to lift fuselage 21 in response to the action of rotor systems 23 (only one shown). Rotor-blades of rotor systems 23 are not shown. Two nacelles 25 (only one shown) each substantially enclose a constant-velocity drive system 27, obscuring constant-velocity drive system 27 from view in FIG. 1. Of course, each rotor system 21 is driven by associated engines (not shown), one engine housed within each nacelle 25.
Referring now to FIGS. 2-6, Zoppitelli et al. teach a differential torque-splitting mechanism fitted to a rotor mast, for driving in rotation the hub of a convertible aircraft tilting rotor, as described above with reference to FIG. 1.
In FIGS. 2-6, mast 29 of the rotor, driven by its base (not shown) in rotation about its longitudinal axis Z-Z, supports, a differential mechanism, designated as a whole by number 31. This mechanism 31, which belongs to the means for constant-velocity drive of the rotor hub, mainly comprises an assembly of three discs coaxial about the axis Z-Z and placed one on top of the other along this axis, a central disc 33 of which is arranged axially between the other two discs 35 and 37, one of which, arranged axially between central disc 33 and a seating shoulder 39, annular, peripheral and projecting radially toward the outside on shaft or mast 29, is termed an inner disc 35, as it is arranged along the axis Z-Z at the base end of mast 29, and therefore toward the inside of the convertible aircraft structure, while third disc 37, termed the outer disc, is arranged axially between central disc 33 and an axial preload device 41, fitted along a threaded portion of mast 29, to provide axial stacking (along Z-Z) of the three discs 33, and 37 of the assembly with preloading, under the conditions and for the reasons which are explained below.
Central disc 33 is made integral in rotation with mast 29 by internal axial splines 43 in its central bore, which are engaged with external axial splines on a cylindrical splined portion 29a of mast 29, to transmit the torque. As can also be seen in FIG. 7, central disc 33 has a central portion 45, between two cylindrical journals 47 and 49 at the axial ends, which is extended radially toward the outside by four spider arms 51 each drilled with two cylindrical bores 55 side by side and with parallel axes. The four spider arms 51 are diametrically opposite in twos, and regularly distributed over the periphery of central portion 45 of central disc 33.
Each of the inner 35 and outer 37 discs comprises a peripheral portion respectively 57 and 59, which is offset axially toward central portion 45 of central disc 33 and surrounds inner axial journal 47 (the lower one in the drawings) or respectively outer axial journal 49 (the upper one in the drawings) of the latter, and each of the peripheral portions 57 and 59 respectively of inner disc 35 and of outer disc 37 also has, projecting radially toward the outside, four spider arms respectively 61 and 63, also diametrically opposite in twos and regularly distributed over the periphery of said peripheral portions 57 and 59, and each also drilled with two bores respectively 65 and 67 side by side and with parallel axes, and of the generally the same diameter as bores 55 in central disc 33.
Moreover, inner disc 35 supports two drive pins 69, of generally cylindrical shape with a circular cross-section, with axes contained within a radial (relative to the axis Z-Z) plane, and which project toward the outside of the inner disc and occupy diametrically opposite positions, each being between two spider arms 61 of disc 35, and at the same time offset axially toward central portion 45 of central disc 33, so that they can be housed in one of the cut-away portions, delimited at the periphery of this central portion 45 of central disc 33, between two spider arms 51 of disc 33, (see FIGS. 5 and 6). Similarly, outer disc 37 has two drive pins 71, of the same cylindrical form with a circular cross-section and of the same size as pins 69 and also diametrically opposite and projecting toward the outside of peripheral portion 59 of disc 37, while being at the same time offset axially toward central portion 45 of central disc 33, so that they can each be housed in one of the four cut-away portions delimited by spider arms 51 on the periphery of central disc 33 and alternating in a circumferential direction about the axis common to these three discs 33, 35 and 37, with drive pins 69 of inner disc 35.
The three discs 33, 35 and 37 are placed one on top of the other axially so that at rest spider arms 51, 61 and 63 are directly above each other, and bores 55, 65 and 67 aligned between one disc and another, as shown in the left-hand half-view in FIG. 4, so that, in each of the eight groups of three bores 55, 65 and 67 aligned in this way, there can be housed one respectively of eight connecting pins 73, distributed in this way, over the periphery of the three discs, in four assemblies of two adjacent connecting pins 73, radially at the same distance from the axis Z-Z of mast 29, and distributed regularly in four pairs of connecting pins 73, diametrically opposite in twos and along two diametral planes perpendicular to each other, as shown in FIG. 2.
Each connecting pin 73 has its longitudinal geometrical axis A-A substantially parallel to the axis Z-Z of mast 29, and is hinged in each of the three corresponding spider arms 51, 61 and 63 by one respectively of three ball joint connections 75, 77 and 79 which are centered on the axis A-A. As shown in the right-hand half-view in FIG. 4, each connecting pin 73 is a pin with triple ball joints, with a central ball joint 81 with a larger diameter than that of two end ball joints 83, of the same diameter, each of ball joints 81 and 83 being a laminated ball joint retained radially (relative to the axis A-A) inside a cylindrical laminated bearing 85 (for the central ball joint connection 75) and 87 (for each of the end ball joint connections 77 and 79), cylindrical laminated bearings 85 and 87 being substantially coaxial about the geometrical axis A-A of corresponding connecting pin 73. For this reason, each connecting pin 73 is in the form, viewed from the outside, of a cylindrical sleeve divided axially into three parts placed one on top of the other and slightly spaced apart from each other, with a radial collar at the upper end (see FIG. 7) and each enclosing three ball joint connections 75, 77 and 79 offset along the axis A-A.
After the eight connecting pins 73 are installed, central disc 33, integral in rotation with mast 29, is a driving disc for inner disc 35 and outer disc 37, which are driven discs of mechanism 31, and each of which can drive in rotation, about the axis Z-Z, and by its two corresponding drive pins 69 or 71, at least one of driving devices connected to the hub to cause the latter to rotate, which are each hinged to the hub, so as to drive the latter in rotation, from the rotation of mast 29.
For the reasons explained below, in order to allow relative rotation, about the axis Z-Z of rotation of mast 29, between each of driven discs 35 and 37, on the one hand, and on the other, driving disc 33 and mast 29, each of driven discs 35 and 37 is mounted, in its portion which surrounds mast 29, axially between two radial annular bearings 89, surrounding mast 29 and substantially coaxial about the axis Z-Z of the latter. Thus the central portion of driven discs 35 is fitted between an inner radial bearing 89, seated against shoulder 39 of mast 29, and an outer radial bearing 89 seated against the inner axial end of journal 47 of driving disc 33, while the central portion of the other driven disc 37 is fitted between a radial bearing 89, seated against the outer end face of journal 49 of driving disc 33, and another radial bearing 89 with loads applied axially, in the direction which applies axial preloading to the stack of three discs 33, 35 and 37 and of four bearings 89, by axial preload device 41 which, in these drawings, is shown schematically as consisting of a nut 91 screwed around the externally threaded portion 29b of mast 29.
In addition to radial annular bearings 89, which may be plain but are preferably each a cylindrical laminated bearing, as shown, or possibly truncated cone-shaped, comprising at least one vulcanized elastomer washer between two metal washers, two axial bushings 93 are provided to facilitate relative rotation between each of driven discs 35 and 37, on the one hand, and on the other mast 29 and driving disc 33. One of two bushings 93 is fitted between peripheral portion 57 of driven disc 35 and journal 47 of driving disc 33, while the other axial bushing 93 is fitted between peripheral portion 59 of other driven disc 37 and other journal 49 of driving disc 33. These two axial bushings 93 are also substantially coaxial about the axis Z-Z of mast 29.
In FIGS. 2-6, differential mechanism 31 is such that two drive pins 69 of driven disc 35 are not only diametrically opposite relative to the axis Z-Z, but project radially toward the outside of driven disc 35, perpendicularly to the axis Z-Z, and coaxial about a first diametral axis X-X of mechanism 31 and of mast 29, so that pins 69 constitute a first diametral drive arm integral with driven disc 35. Similarly, the two drive pins 71 of driven disc 37, also diametrically opposite relative to the axis Z-Z and perpendicular to the latter, overhanging and projecting radially toward the outside of driven disc 37, and coaxial about a second diametral axis Y-Y of mechanism 31 and which at rest is perpendicular to the first diametral axis X-X and converging with the latter on the axis Z-Z, constitute a second diametral drive arm, integral in rotation with driven disc 37 and, when mechanism 31 is at rest, perpendicular to the first diametral drive arm formed by pins 69.
This differential mechanism 31 is compatible with a double-gimbal device 96, as shown in FIGS. 7 and 8, for a rotor in which this double-gimbal device 96 constitutes both the driving means and the tilting means placed between differential mechanism 31 on the one hand and, on the other, a rotor hub supporting blades, and which is thus mounted so as to pivot about any flapping axis intersecting the axis Z-Z of mast 29 and extending in any direction about this axis Z-Z, so that the hub, and therefore the rotor, can be driven in rotation about a geometrical axis inclined in any direction about the axis Z-Z of mast 29.
Referring now to FIGS. 7 and 8, double-gimbal device 96 comprises a first gimbal 97, substantially in the shape of an octagon (viewed in plan) mounted so as to pivot relative to mast 29 by two first bearings 101a, 101b which may be plain cylindrical bearings or, preferably, bearings consisting of cylindrical, conical, and/or where appropriate spherical laminated elements. A second gimbal 99, also substantially octagonal in shape, and arranged above first gimbal 97, is mounted so as to pivot in a similar manner by two second bearings such as 103a (the other one is not visible), of the same type as bearings 101a and 101b so that second gimbal 99 can pivot relative to mast 29.
The two gimbals 97 and 99 are thus each driven in rotation by one respectively of driven discs 35 and 37, themselves driven by mast 29 and driving disc 33, about the axis Z-Z of mast 29, while being mounted so as to pivot each about one respectively of the two axes, normally perpendicular, X-X and Y-Y.
In addition, the first gimbal 97 is hinged to a casing or hub body by two first ball joint connections such as 107a (see FIG. 8), preferably comprising laminated ball joints, each combined with a cylindrical or conical laminated bearing, and which are diametrically opposite relative to the axis Z-Z of mast 29, and each centered on the second diametral axis Y-Y, being retained in two small sleeves 105 coaxially about the axis Y-Y on gimbal 97, in the neutral or rest position of the rotor, the two first ball joint connections such as 107a remaining centered substantially in a diametral plane, defined by the axis Z-Z and by the second diametral axis Y-Y, when first gimbal 97 is pivoted about the first diametral axis X-X.
In a similar manner, second gimbal 99 is hinged to a hub body by two second ball joint connections 109a and 109b, also preferably comprising laminated ball joints combined with cylindrical or conical laminated bearings and, diametrically opposite relative to the axis Z-Z and each centered, at rest or in the neutral position of the rotor, on the first diametral axis X-X, while being retained in small sleeves 111 coaxial about the axis X-X on gimbal 99, these second ball joint connections 109a and 109b remaining substantially centered in a diametral plane defined by the axis Z-Z and the first diametral axis X-X when second gimbal 99 is pivoted about the second diametral axis Y-Y.
In this embodiment, a rotor hub is connected to mast 29 by two crossing gimbals 97 and 99, hinged to the inside of the hub by ball joint connections, preferably laminated such as 107a and 109a, 109b, and hinged so as to pivot about the two perpendicular diametral drive arms 69-69 and 71-71, at rest, by bearings 101a, 101b and such as 103a, according to an arrangement at the same time constituting a mechanism for tilting the hub and the blades, allowing pivoting of the hub as a whole about any flapping axis intersecting the axis Z-Z of mast 29 and running in any direction about the axis Z-Z, and a mechanism giving constant velocity drive of the hub and of the blades about a geometrical axis of rotation of the hub, which may be inclined in any direction about the axis Z-Z of mast 29 by causing gimbals 97 and 99 to pivot about their respective diametral axes X-X and Y-Y. The torque is transmitted between mast 29 and the hub by two transmission trains each comprising mast 29, the central disc 33, one respectively of the driven discs 35 and 37, and therefore gimbal 97 or 99 pivoting on driven disc 35 or 37, the corresponding two bearings 101a, 101b or such as 103b, the corresponding two ball joint connections such as 107a or 109a, 109b and the hub.
With a pivoting device of this type with two gimbals 97 and 99, it is known that tilting of the rotor disc and therefore of the hub relative to the axis Z-Z of mast 29 induces a cyclic relative rotation of these two gimbals 97 and 99, at a frequency of 2Ω (where Ω is the frequency of rotation of the rotor), the two gimbals 97 and 99 performing rotation movements in opposite directions and of equal amplitude about the drive axis and in a plane perpendicular to this drive axis. The differential mechanism 31 compensates kinematically for this cyclic relative rotation of the two gimbals 97 and 99, by means of the connecting pins 73, linking driven discs 35 and 37 to driving disc 33, and which are inclined slightly while accompanying the rotation of driven discs 35 and 37 in opposite directions about the axis Z-Z of mast 29. At the same time, the static torque transmitted by mast 29 to two gimbals 97 and 99 is split by driving disc 33 between two driven discs 35 and 37, by means of connecting pins 73. This capability of the differential mechanism 31 to allow any relative movement of two gimbals 97 and 99 in the plane perpendicular to the drive axis eliminates the hyperstatic characteristics of a device in which the tilt mechanism with two gimbals would be directly connected to mast 29.
The constant velocity characteristics are thus obtained by the kinematic compatibility between the tilting and drive means using two gimbals 97 and 99, by means of differential mechanism 31.
Transmission of the loads from the rotor (lift and coplanar loads) to mast 29 is provided, from the hub to mast 29, via two gimbals 97 and 99 which, in opposite directions, transmit the torque from mast 29 to the hub. The radial annular bearings 89 and axial bushings 93, allowing relative rotation between driven discs 35 and 37 (connected to gimbals 97, 99) and driving disc 33 connected to mast 29, assist in transmitting the lift load and the coplanar loads, the lift also being transferred through the presence of axial preload device 41 with elastic deformation of the stack of three discs 33, 35 and 37 and of four annular radial bearings 89 against shoulder 39 on mast 29.
While the constant-velocity drive systems taught by Zoppitelli et al. may be suitable for smaller, lighter, less powerful rotary-wing aircraft, significant limitations become apparent when the constant-velocity drive systems taught by Zoppitelli et al. are considered for use in larger, heavier, more powerful rotary-wing aircraft. For example, in order to increase the torque transfer capability of a constant-velocity drive system taught by Zoppitelli et al., the overall size of the torque-splitting mechanism would necessarily increase. Additionally, since the two-gimbal device associated with the torque-splitting mechanism substantially envelopes the torque-splitting mechanism, the overall size of the two-gimbal device would also necessarily increase. It is desirable to configure the rotating components of rotor systems to remain as close to the axis of rotation of the mast as possible to minimize undesirable resultant forces. Clearly, increasing the size of the torque-splitting mechanism and the two-gimbal device taught by Zoppitelli et al. is not desirable and does not provide a satisfactory solution for providing a constant-velocity drive system for a larger, heavier, more powerful rotary-wing aircraft.
While the above described rotor hub advancements represent significant developments in rotor hub design, considerable shortcomings remain.